This invention relates to a turbofan engine, and more particularly, the invention relates to managing the turbofan engine's fan operational line.
A typical turbofan engine includes low and high spools. The low spool is coupled to a fan or turbofan and typically supports a low pressure turbine and compressor. The spools, turbine and compressor are housed in a core nacelle. The turbofan is arranged upstream from the core nacelle. A fan nacelle surrounds the turbofan and core nacelle to provide a bypass flow path. Airflow enters the engine through an inlet cowl provided by the fan nacelle before reaching the turbofan.
Maintaining aerodynamic stability of the fan in large commercial high bypass turbofan engines is a significant factor for overall engine performance. The fan operating characteristics are set to maintain sufficient fan stability margin at all operating conditions. One of the important performance limitations for the fan is the loss in fan stability margin that occurs during aircraft maneuvers and cross-wind operating conditions. During a maneuver or in a cross-wind, the inlet flow approaches the aircraft inlet in a direction that is not inline with the inlet axis of the engine. This can cause local pressure gradients and flow separation on the inlet cowl lip that results in significant pressure distortion entering the fan.
The increase in inlet distortion results in the loss of fan stability margin. If the distortion is excessive, the resulting loss in stability margin can lead to instability, stall or flutter of the fan. Therefore, the nominal fan operating line must be set such that sufficient stability margin is maintained, which typically causes the operating line to be lower than desired for overall engine performance throughout the flight envelope.
What is needed is a fan operating line provided such that sufficient fan stability margin is maintained without compromising the overall engine performance.